In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Air compressors deliver a source of compressed air that may perform many useful functions. One example of where air compressors are used is for drilling rigs. Although the explanation that follows is limited to drilling rigs, it should be understood that the disclosed air compressor system and methods of operation thereof are not limited to drilling rigs. Some drilling rigs operate as follows. A drill bit of a drill string (which is one or more drill pipes connected together) is rotated to drill a hole in the ground, i.e., in earth and/or rock. In order to flush the cuttings from the hole as it is being drilled, an air compressor may be used to deliver pressurized air which is communicated downwardly through the drill string to the front face of the drill bit. The cuttings get caught in the airflow from the drill bit and are brought to the surface as the air travels upwardly along the exterior of the drill string. The pressurized air may also serve to cool the cutting elements of the drill bit. This is one way compressed air may be used by drilling rigs.
Compressed air may also be used in percussive drilling where the compressed air is used to reciprocate an impact piston which applies percussive blows from a piston to a rotating drill bit to enhance the cutting action. The piston may be disposed below the ground surface immediately above the drill bit (i.e., a so-called down-the-hole hammer), or it may be disposed on above the surface of the drill hole.
In many compressed air applications it is common to drive the air compressor by a engine (for example a fuel-driven engine or an electrically driven motor), which may also drive other equipment, such as a hydraulic system which may function to perform the following functions: power hydraulic systems to raise and lower the drill string, rotate the drill string via a gearbox, add drill rods to the drill string as drilling progresses, remove drill rods from the drill string as the drill string is being withdrawn from the hole, raise and lower a drilling mast, raise and lower leveling jacks, and propel the drilling rig (in the case of a mobile drilling rig). The engine also may drive a hydraulic pump and a cooling fan of a cooling system.
The compressed air needs of such a drilling machine are associated with the supplying of flushing air for flushing cuttings and/or driving the impact piston of a percussive tool and/or other accessories that may be used by the drilling rig. During operation of the drilling rig, there may be no need for pressurized air, such as during the adding or removal of drill rods, relocating the drill rig, setting up the drill rig, lunch breaks. Although there is no need during those periods to circulate compressed air to flush cuttings or to reciprocate the impact piston, it still may be necessary to drive the engine (that drives both the air compressor and the hydraulics) in order to continue to power the hydraulics.
In some air compressing systems, the drive connection between the air compressor and the engine is such that the air compressor is driven whenever the engine is driven, despite the fact that continuous operation of the air compressor is not necessary when drilling is not taking place.
There are certain measures that could be taken to further reduce the unnecessary consumption of energy. For example, a clutch could be provided between the engine and the air compressor to unload the compressor during periods of low air requirements, but that would add considerable cost to the equipment, and the clutch would rapidly wear in situations where the compressor has to be unloaded frequently. Additionally, it is uneconomical and impractical to switch the compressor on and off at frequent intervals. Moreover, even during periods where a large quantity of compressed air is not needed, smaller quantities may still be needed, so that the air compressor may have to cycle on and off to keep an air reservoir (a place where pressurized air from the air compressor may be stored) sufficiently pressurized for the smaller quantities.
Another possible energy-saving measure involves the provision of a variable speed gear drive for unloading the air compressor, but such a drive is complicated and relatively expensive, as would be a two-speed gear drive with clutches. With a variable speed gear drive, the revolutions per minute (RPMs) from the motor that are driving the air compressor could be reduced for reduced energy consumption.
Another possible measure involves driving the air compressor with a hydraulic motor that can be easily be stopped or slowed during periods of low pressure requirements. For example, when a drill rod is being added to the drill string. However, such drives are relatively inefficient (many are at most 80% efficient), so any energy savings realized during periods of low compressed air consumption would likely be lost during periods of high air compressed consumption.
Therefore, it would be desirable to provide an air compressing system employing an engine-driven air compressor which is energy efficient.